In the description that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicants expressly reserve the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.
X-rays occupy that portion of the electromagnetic spectrum between approximately 10−8 and 10−12 m. Atoms emit x-rays through two separate processes when bombarded with energetic electrons.
In the first process, high-speed electrons are decelerated as they pass through matter. If an individual electron is abruptly decelerated, but not necessarily stopped, when passing through or near the nuclear field of a target atom, the electron will lose some of its energy which, through Plank's law, will be emitted as an x-ray photon. An electron may experience several such decelerations before it is finally stopped, emitting x-ray photons of widely different energies and wavelengths. This process produces the bulk of x-ray radiation and results in a continuous-type spectrum, also called Bremsstrahlung.
In the second process, an incident electron collides with and ejects an orbital electron of a target atom. If the ejected electron is from an inner shell orbit, then an electron in an outer shell orbit will fall to the inner vacant orbit with an attendant emission of an x-ray photon. In this process energy is emitted in the form of an x-ray whose energy or wavelength represents the orbital transition involved. Because the energies of orbital electrons are quantized, the x-ray photons emitted are also quantized and can only have discrete wavelengths characteristic of the atom. This gives rise to their classification as characteristic x-rays.
Several methods have been used to produce the incident electrons at a cathode and accelerate them into a target anode. One traditional approach has been the use of an x-ray tube. Depending upon the method used in generating the electrons, x-ray tubes may be classified in two general groups, gas tubes and high-vacuum tubes.
FIG. 1 shows a conventional gas x-ray tube. The x-ray generating device 110 is substantially made of a glass envelope 120 into which is disposed a cathode 125 which produces a beam of electrons 140 which strike an anode 130 thereby causing x-rays to be emitted 150 which can be used for sundry purposes including medical and scientific. The cathode is powered by a high voltage power supply via electrical leads 135. In addition, a gas pressure regulator 115 regulates the gas pressure in this type of x-ray device.
High vacuum tubes, an example of which is shown in FIG. 2, are a second type of x-ray tube. FIG. 2 shows a vacuum x-ray tube device with a thermionic cathode. In this type of device 210 a glass envelope 220 serves as the vacuum body. The cathode 225 is deposed within this vacuum and is provided with electrical leads 235. Electrons 240 are emitted by thermionic emission from the cathode 225 and strike an anode target 230 The efficiency of such emission of x-rays is very low causing the anode to be heated. To increase the lifetime of this device, it has been necessary to provide a cooling mechanism. One embodiment of a cooling mechanism is a chamber 260 through which water is circulated by the use of an inlet 265 and an outlet 270. To improve the efficiency of the emitted beam of electrons a focusing shield 245 is often utilized. The focusing shield 245 collimates the thermionically emitted electrons and directs them to the anode 230. However, the thermionic origin of the electrons makes focusing to a small spot size difficult. This, in part, limits the resolution of modern x-ray imaging (see, for example, Radiologic Science For Technologist, S. C. Bushong, Mosby-Year Book, 1997). X-rays 250 emitted from the anode 230 pass through a window 255 and are subsequently available for sundry purposes, including medical and scientific. An additional feature of this type of device is an exterior shutter 275. It has been found necessary to incorporate such a shutter to prevent the incidental emission of x-rays associated with the heating decay of the cathode. This is because even though the application of power to the cathode may be terminated, residual heating may be such that electrons continue to be emitted towards the target and continue to produce x-rays.
This process of x-ray generation is not very efficient since about 98 percent of the kinetic energy of the electron stream is converted upon impact with the anode into thermal energy. Thus, the focus spot temperature can be very high if the electron current is high or continuous exposure is required. In order to avoid damage to the anode it is essential to remove this heat as rapidly as possible. This can be done by introducing a rotating anode structure.
As noted above, a shutter (e.g. 275) is necessary in such devices because thermionic emission of electrons from a cathode does not allow for precise step function initiation and termination of the resulting electron beam. Indeed, while still at elevated temperatures and subsequent to removal of power, a thermionic cathode may emit electrons which may cause unwanted x-ray emission from the target. In operation the shutter is held open either mechanically or by means of a micro switch.
Moreover, due to high temperature heating, the cathode filament has a limited lifetime, typically around a few hundred hours in medical applications and thousand hours in analytical applications. Under normal usage, the principle factor determining the lifetime of the x-ray tube is often damage to the cathode filament.
The amount of useful x-rays generated in the anode is proportional to the electron beam current striking at the anode. In thermionic emission, the electron beam current is only a small fraction of the current passing through the cathode filament (typically {fraction (1/20)}). In modern medical applications such as digital radiography and Computed Tomography (CT), very high x-ray intensity is required concomitantly requiring a very high thermionic emission cathode current. Therefore, a principle limitation in these applications is the amount of electron beam current generated by the cathode.
A possible improvement in the generation of x-rays is the introduction of field emission cathode materials. Field emission is the emission of electrons under the influence of a strong electric field. However, the incorporation of conventional field emission cathode materials into x-ray generating devices presents certain challenges. For instance, the field emission cathode materials must be capable of generating an emitted electron current density of a sufficiently high level (can be as high as 2000 mA on the target for medical applications) such that, upon striking the anode target material, the desired x-ray intensity is produced.
Many conventional field emission materials are incapable of producing the desired emitted electron circuit density absent the application of a relatively high electrical field to the cathode. Moreover, many of the conventional field emission materials cannot produce stable emissions at high current densities under high applied electrical fields. The use of high control voltages increases the likelihood of damaging the cathode material, and requires the use of high powered devices which are costly to procure and operate.
Conventional field emission materials such as metals (such as Mo) or a semiconducting materials (such as Si), with sharp tips in nanometer sizes have been utilized. Although useful emission characteristics have been demonstrated for these materials, the turn-on electric field is relatively high, typically on the order of 50-100 V/μm at a current density of 10 mA/cm2. (See, for example, W. Zhu et al., Science, Vol. 282, 1471, (1998)).
Carbon materials, in the form of diamond and carbon nanotubes, have emerged as potentially useful for electron field emission materials.
Low-field emission has been observed in diamond-based materials (3-5 V/μm for 10 mA/cm2 current density). However, the emission is unstable above a current density of 30 mA/cm2 and the fabrication of uniform sharp tips is difficult and costly. Moreover, the stability of these materials in a real device environment is of concern, partially due to ion bombardment, reaction with chemically active species and high temperatures. (See, for example, I. Brodie and C. A. Spindt, “Advances in Electronics and Electron Physics”, edited by P. W. Hawkes, Vol. 83, 1 (1992)).
The use of diamond materials as field emitters also suffers from the problem that the diamond produces lower than desired current densities. While observations of localized emission hot spots have been reported as having a current density on the order of 100 A/cm2, the actual emission areas have not been measured, neither are they understood or reproducible. See, e.g. K. Okano et al., Applied Physics Letters, Vol. 70, 2201 (1997), which is hereby incorporated by reference in its entirety. Diamond emitters and related emission devices are disclosed, for example, in U.S. Pat. Nos. 5,129,850; 5,138,237; 5,616,368; 5,623,180; 5,637,950; 5,648,699; Okano et al., Applied Physics Letters, Vol. 64 2742 (1994); Kumar et al., Solid State Technologies, Vol. 38, 71 (1995); and Geis et al., Journal of Vacuum Science Technology, Vol. B14, 2060 (1996), all of which are hereby incorporated by reference in their entirety.
The previously published studies of carbon nanotube emission materials have reported relatively low current densities, typically on the order of 0.1-100 mA/cm2. The higher reported electron emission data are difficult to interpret and are unreliable because, for example, the data is independent of the distance between the emitting cathode and target anode material (U.S. Pat. No. 6,057,637). Carbon nanotube emitters are disclosed, for example, in T. Keesmann in German Patent No. 4,405,768; Rinzler et al., Science, Vol. 269, 1550 (1995); De Heer et al., Science, Vol. 270, 1179 (1995); Saito et al., Japan Journal of Applied Physics, Vol. 37, 1. 346 (1998); Wang et al., Applied Physics Letters, Vol. 70, 3308 (1997); Saito et al., Japan Journal of Applied Physics, Vol. 36, 1. 1340 (1997); Wang et al., Applied Physics Letters, Vol. 72, 2912 (1998); and Bonard et al., Applied Physics Letters, Vol. 73, Page 918 (1998), all of which are hereby incorporated by reference in their entirety.
Emissions as high as 4 A/cm2 from single-wall carbon nanotube films deposited on different substrates has been reported (W. Zhu et al., Applied Physics Letters, Vol. 75, 873 (1999), pending U.S. patent application Ser. No. 09/259,307). The threshold field for 10 mA/cm2 current density is <5 V/μm (C. Bower et al., in “Amorphous and Nanostructured Carbon”, edited by J. Sullivan, J. Robertson, O. Zhou, T. Allen, and B. Coll, Materials Research Society Symposium Proceeding, Vol. 593, Page 215 (2000)).
X-ray tubes used for medical applications usually contain dual focus spots with “apparent” spot sizes of 0.3 mm2 and 1 mm2. With a target angle of 6° and 15°, this corresponds to an actual area of electron bombardment of 0.3×3 mm2 and 1×4 mm2. Further reduction of the focus spot requires a smaller target angle and a higher electron current. This is not possible due to constraints of power supplied to the cathode filament.
Another difficulty with a conventional thermionic emitter is the space charge effect. The space charge is very sensitive to applied x-ray voltage (kV) and filament current. Thus, it is difficult to achieve independent control of electron beam current (mA) and kV unless the tube is operating in the so called saturation limit. This generally implies larger mA for higher kV.
In digital fluoroscopy and radiography, an energy subtraction technique (where the image obtained with a lower average x-ray energy is subtracted from that produced by a higher x-ray energy) is used to enhance the contrast of certain materials (such as contrasting agent iodine). See, for example, Radiologic Science for Technologist, S. C. Bushong, Mosby-Year Book, 1997. This requires alternating high and low kV in every data point acquired. Due to the inherent difficulty of initiation and termination of thermionic electron sources, the process is slow and patients are exposed to unnecessary higher dosages of x-rays.
In computed tomography, the uniformity of the x-ray fan beam is crucial. Achieving uniformity in conventional tube design is difficult because emission of x-rays from the target surface is anisotropic (namely, dependent on the emission direction relative to the surface). Different parts of the x-ray beam come from different combinations of emission angles from different parts of the focus area. Thus, even when the focus spot is bombarded with an uniform electron beam the resulting x-ray beam is non-uniform.
Therefore, it would be desirable to construct x-ray generating devices which incorporate field emitting cathode materials capable of reliably producing high emitted electron current densities, without reliance upon thermionic emissions, or high control or externally applied voltages. It would also be desirable to provide x-ray generating devices with an emitted electron beam that is easy to control and focus.